1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle. More particularly, the present invention relates to a steering apparatus for a vehicle, having a stem pipe pivotally mounted in a head pipe by upper and lower bearings, and secured with a top bridge using a single fastening bolt.
2. Description of the Background Art
There is a known steering apparatus for a vehicle, which has a stem pipe as an essential component thereof pivotally mounted in a head pipe by upper and lower bearings, mounted on the vehicle, such as, a two-wheeled motorcycle or the like. An example of such steering apparatus is disclosed in the Japanese Patent Laid-Open No. Hei 11-165684 (see FIG. 1 thereof, reproduced herein as FIG. 6 of the present application).
As shown in FIG. 6 of the drawings (corresponding to FIG. 1 of Japanese Patent Laid-Open No. Hei 11-165684), one example of a known steering system (1) includes a stem pipe (5) inserted in a head pipe (4) with upper and lower bearings (6, 7) interposed therebetween. A bottom bridge (9) is coupled to a lower portion of the stem pipe (5), and a top bridge (8) is coupled to an upper portion of the stem pipe (5). A front fork (100) is attached to the top bridge (8) and the bottom bridge (9).
The upper bearing (6) is preloaded by an axial force that is applied from above by a nut (17A). Since the nut (17A) tends to loosen due to vibrations, etc., such loosening of the nut (17A) is prevented by an axial force applied thereto by a locking nut (17B). Such conventional locking structure with two nuts (17A, 17B) is called a double nut structure.
Although the double nut locking structure disclosed in the Japanese Patent Laid-Open No. Hei 11-165684 is widely used, such locking structure is made up of many components because such locking structure requires the two nuts (17A, 17B).
In order to install the two nuts (17A, 17B), a sufficient length from the upper bearing (6) to the top bridge (8) is required, resulting in a longer distance by which the stem pipe (5) projects from the upper bearing (6). Therefore, external forces in directions perpendicular to the axis are applied to the upper end of the stem pipe (5) through the top bridge (8). The bending moment on the stem pipe (5) is greater as the external forces are greater. Also, the bending moment on the stem pipe (5) is greater as the distance by which the stem pipe (5) projects from the upper bearing (6) is greater. For reducing the bending stress of the stem pipe to a certain value or smaller, it is necessary to increase the bending rigidity and the wall thickness of the stem pipe (5).
Furthermore, since the stem pipe (5) needs to be externally threaded for installing the two nuts (17A, 17B) thereon, the cost of externally threading the stem pipe to receive such plural nuts is high.
However, small-size vehicles, particularly two-wheeled motorcycles, are required to be made up of a reduced number of components. Also, the two-wheeled motorcycles generally include a stem pipe having a reduced wall thickness.
The present invention has been made to overcome such drawbacks of the known steering apparatus. Accordingly, it is one of the objects of the present invention to provide a steering apparatus for vehicle which is made up of a reduced number of parts, and which includes a stem pipe having a reduced wall thickness.